Carburizing salt bath



Patented July 24, 1951 Edna L. Sawyer, Stamford, and Northrup W. Marr,Springdale, Conn., assignors to American Cyanamid Company, New York, N.Y., a corporation of Maine No Drawing.

Thepresent invention relates to salt mixtures which, when in a moltencondition at elevated temperatures, are adapted for carburizing ferrousmetal, a bath made from such salt mixtures, and a process of using thebath.

,It s an object of this invention to provide a case carburizing baththat will give efiective case depths of at least 16 mils in one hour at1600 F. consistently, but which, at the same time, will not leave aninsoluble sludge on the article after it is removed from the bath, oilquenched, and shaken in hot water. It is a further object of the presentinvention to provide an easy washing carburizing bath operable over atemperature range of 1300 to 1750 F. It is a still further object toprovide an inexpensive, novel, and efiective activator for such a bath,namely, an alkali ii'ietalsilicate. Still other objects will be obvioufrom the description of the invention given below.

A cyanidin bath containing unactivated sodium cyanide does not coat thework with insoluble sludges, but is unsuitable as a carburizing bathbecause it carburizes slowly and gives effective casedepths ofonly'about 12 mils at the highest temperature at which it can beoperated, namely, around 1550 F., and is further unsatisfactory as acarburizing means since it nitrides the Workas well as carburizes it,the case at the surface analyzing about 0.6% C and about 0.5%N.

1 The usual liquid carburizing bath provides sat Application November29, 1950, Serial No. 198,226

I soiai s'l (Cl. 148-27) f and unacceptable level. For example, while thisfactory case depths through the activation of calcium and bariumsalts. Such a bath may give an effective case depth of from 16 mils inanhour at 1600 F. to 125 mils in 16 hours at 1750 F. A disadvantage of abath activated with calcium and barium salts is that these salts sludge,forming oxides and carbonates that adhere to the article when it iswithdrawn from the bath and that cannot be dissolved in hot water, butmust be removed from the article by chemical and/or mechanical means. vV 'A'n'easy washing carburizing bath has now been found thatincorporates the easy washing characteristics of a cyaniding bath withthe high carburizing activity'of a calcium salt-activated bath.

highly alkaline bath might give case depths of 16 mils inthe beginningof its operation, penetration would soon drop to the order of 12-14 milsor less. It was believed that this reduced inactivity was caused bystabilization of the bath by the alkaline materials present. A standardmethod oiv correcting excess alkalinity has been to add acid materialsto the bath. Examples of such acid mate rials are free silica and thesodium acid silicates. Thus corrected, the bath retained its reactivityand gave good carburizing penetration overa long period of time.

The present invention demonstrates that a highly alkaline bath can beprepared containing substantial quantities of alkali metal containing noacidic groups, and that the bath gives consistent high penetration overa long period of time. This combination of high alkalinity and alkalimetal silicatescontaining no acidic elements is unobvious and, so farasthe inventors are aware, there is no suggestion of the combination inthe prior art. In the following description, wherever alkali metalsilicates are mentioned, it is to be understood they are the normalsalts, containing no acidic-hydrogen. They may be added as such orformed in situ in the bath. The bathcontemplated in the presentinvention gives effective case depths on fracture up to about 32 milsduring a four-hour treatment at l600 1?. in the bath, and the. film ofadherent salts after an oil quench is so soluble that it may be removedsimply by shaking the work in hot water a few times. Adherent salts fromholes A; inch indiameter and /2 inch deep are easily removed by i suchmeans. i

The bath of the present invention, unlike a cyf aniding bath, isoperable up to 1750 F. and gives satisfactory case depths even though itcontains no alkaline earth activator. The bath of the present inventionis further characterized by its high activity in commercial-size pots,its ease of maintaining a good cover, low sodium cyanide loss, and lownitriding.

The present. invention involves a completely new concept in carburizingbaths. In the past,

When making up the bath, a-salt mixture with the following range ofingredients may be use (part by weight) Sodium chloride 20 -50 Sodiumcarbonate 25 45., Sodium cyanide 15 35 Anhydrous sodium silicate,sufficient to provide an SiOz content of 0.5- 5.5 Graphite 0.3- 1.0

Various equivalents may be used for the above substances. For example,various other water: soluble cyanogen-forming compounds maybe usedinstead of sodium cyanide, namely, other alkali metal cyanides,cyanates, cyanamides, and the like. Organic cyanogen compounds are alsosuitable as, for example, cyanamide, dicyandiamide, melamine, melam,melem, melon, am: meline, ammelidahydrogen cyanide polymers,

and the like. i The sodium carbonate of the bath may be substituted for,in total or in part, by other alkali metal carbonates, and the sodiumchloride maybe similarly substituted by other alkali metal chlorides.The sodium silicate used,

may vary widely in its NazO:Siz ratio and may be replaced by potassiumsilicate. However, a sodium silicate containing about 50% NazO and 50%Si02, conforming to the approximate molecular formula NazSiOs, ispreferred.

Although the quantity ranges listed above are operable and arecontemplated by the invention, the following composition represents thepreferred materials and their preferred ranges (parts by weight):

NaCN, 20-25. NaaSiOa, 3-10, equivalent to 1.5- parts of containedsilica. NazCOs, 30-35. Graphite, O.3- 1;0. NaCl, remainder to ,give 100parts total.

"The 1.5-5.0 parts of silica in the sodium silicate isf'airly-critical,as -a content of less than 05 part fails to provide the necessary carbonpenetra-tion, while a content of more than 5.5 parts results-inexcessive sludging in the bath. When using the bath in the silica rangeof 154 parts 'oflcontained silica (3-8 parts alkali metal silidate) thebath should be replenished daily to hiaintain a satisfactory carburizingactivity. A- significant advantage of the bath of this invention is itsability to give good case depths ve'r'f aj widerange of temperaturewithout developing detrimental working characteristics. In'- 'the pastit has been customary to use one bath to cover the range l5'001650 F.and a second bath to cover the range 1600-1'750 F. It was found that thefirst bath could not be heated substantially above 1-650 without foamingor sludging or both, and that the second bath, while having excellentcover and sludge characteristics theneighborhood of 1700 would not givegood case depths below 1650" F. In contrast, the bath of the presentinvention gives excellent case depths with good cover performance andlittle or no sludging over a range-of l300-'1'7 50 F. I Itjhas=furtherbeen found that parts carburized infthe bath of the presentinvention have a good clean metallic appearance when Washed after an'c'il'quench. It is common knowledge that parts similarly washed aftertreatment in a barium activated bath are frequently discolored. Anotheradvantage of the present bath over previous --carburizing baths is thatit maybe operated at a temperature as low as 1400 F., at whichtemperature it gives a case depth (microscopic) of 8 mils in 4 hours.This possibility permits carburization of intricate work withoutdistortion...

lhe following example illustrates but doesnot limit the use of the bath.

Example 1 j iA-rniixture of saltsconsisting of 20% sodium cyanide, 30%sodium carbonate, 40% sodium chloride, and 10% sodium silicate (in theform of .a commercial alkaline cleaner) with 0.3% graphite was meltedand heated to the operating temperature.

As the bath melted, the graphite formed a soft blanket which reduced theloss of cyanide and prevented evaporation of the bath while in use.

To maintain the energizing effect, and to compensate for cyanide lossand drag out from the bath, it is desirable but not absolutely necessaryto add to the bath, in the amount of 78% of the total bath weight, everyworking day, a mixture consisting of 10% sodium silicate, 40% sodiumcyanide, 50% sodium chloride and 0.3% graphite.

Case Depths Obtained on y." Dia. SAE 1020 Steel Rods on Fracture, inMilS Treatment No.

Time, Hours Temp.,

Essentially, case penetration is equal to that produced using a bariumcarburizing salt bath.

This is a continuation-in-part of applicants"? co-pending Serial No.89,576, filed April 25, 1949, now abandoned.

The sodium silicate used in the herein examples contained about 50%silica.

Example 2 Carburizing comparable to that of Example is obtained with abathmade upfrom a mixture consisting of 29% sodium cyanide, 39% sodiumcarbonate, 29% sodium chloride, 3% sodium silicats, and 0.2% graphitereplenished daily with a mixture consisting of 40% sodium cyanide,.28%-sodium carbonate, 29% sodium chloride, 3% sodium silicate, and 0.2%graphite. The bath so used avoids a slight .sludging occasionallyemcountered in the higher silicate ranges ,of the bath .of Example 1. 7

While the invention 5113.5 been described with particular reference tospecific embodiment, it is to :be understood that it is not to .belimited thereto but is to be construed broadly and re. stricted solelyby the scope of the appended claims. 1 We claim:

1. A mixture adapted upon fusion for the car; burizing of ferrous metalarticles which consists essentially of alkali metal chloride about.2.050 parts, cyanogen compound about 15-35 parts, anhydrous alkalimetal carbonate about 25-45 parts, alkali metal silicate equivalent toabout 0.5-5.5 parts of silica, and graphite about 0.-1-1'.0 part, theparts being by weight. i

2. A mixture adapted upon fusion for the carburizing of ferrous metalarticles which consists essentially of sodium chloride about 40 parts,sodium cyanide about 20 parts, sodium carbonate about 30 parts,anhydrous sodium silicate equi aa lent to about 5 parts of silica andgraphite about 0.3 part, the parts being by weight.

3. A mixture adapted upon fusion for the carburizing of ferrous metalarticles which 'con-' sists essentially of alkali metal carbonate about39 parts, anhydrous alkali metal silicate equivalent to about 1.5 partsof silica, alkali metal-cyanideabout .29 parts, graphite about 0.3partand alkali met-a1 chloride to make parts, the parts being by weight.

EDNA L. .SAWYER. NORTI-IRIJP W. ,MARR.

REFERENCES CITED The following references are of record :in file of thispatent:

Number "$5 385

1. A MIXTURE ADAPTED UPON FUSION FOR THE CARBURIZING OF FERROUS METALARTICLES WHICH CONSISTS ESSENTIALLY OF ALKALI METAL CHLORIDE ABOUT 20-50PARTS, CYANOGEN COMPOUND ABOUT 15-35 PARTS, ANHYDROUS ALKALI METALCARBONATE ABOUT 25-45 PARTS, ALKALI METAL SILICATE EQUIVALENT TO ABOUT0.5-5.5 PARTS OF SILICA, AND GRAPHITE ABOUT 0.1-1.0 PART, THE PARTSBEING BY WEIGHT.